Transmissible gastroenteritis virus (TGEV) causes a fatal diarrheal disease in neonatal piglets because it selectively infects and destroys the small-intestinal enterocytes required for nutrient absorption and fluid regulation (H. W. Moon, J. Am. Vet. Med. Assoc., 172, 443 (1978)). Additionally, TGEV replicates in porcine respiratory tract tissues but this does not result in primary respiratory disease (L. J. Kemeny et al., Cornell Vet., 65, 352 (1975)).
A new porcine respiratory coronavirus, tentatively designated PRCV, which has a close antigenic relationship to TGEV, suddenly emerged in 1983 to 1984 and spread within less than 2 years in most if not all European countries, where it now persists enzootically. A cytopathic agent has been isolated in cell culture and pathogenesis studies have shown that it replicated at a high titre in the respiratory tract, but to a very low extent in the gut (M. Pensaert et al., Veterinary Quarterly, 8, 257 (1986)).
Conventional serological tests do not distinguish between PRCV- and TGEV-infected animals. However, antigenic dissimilarities were evident on examining the heterologous reactivity of monoclonal antibodies (MAbs). PRCV was found to be non-reactive towards several non-neutralizing anti-TGEV MAbs directed against the S (spike) or M (membrane) proteins (P. Callebaut et al., J. Gen. Virol., 69, 1725 (1988); J. Garwes et al., Veterinary Record, 122, 86 (1988)). Neutralization-mediating epitopes unique to TGEV have been identified also, e.g., by H. Laude et al., Journees de la Recherche Porcine en France, 20, 89 (1988). Conversely, anti-PRCV S-specific MAbs have been shown to differentiate between the two viruses.
Both types of viruses have common antigenic determinants in the three structural proteins: spike (S), membrane (M), and nucleoprotein (N). The absence of two antigenic sites in the S protein of the PRCV isolates has been the base for their differentiation from the enteric viruses by C. M. Sanchez et al., Virology, 174, 410 (1990). Sequencing of the S gene of a French PRCV isolate by D. Rasschaert et al., J. Gen. Virol., 71, 2599 (1990), and of a 200-nucleotide (nt) fragment of the S gene of a North American PRCV isolate by R. D. Wesley et al., J. Virol., 65, 3369 (1991) has revealed that both S proteins contain, at comparable locations within the protein, a single deletion of 224 and 227 amino acids, respectively.
More recently, C. M. Sanchez et al., in Virology, 190, 92 (1992) determined the genetic relationship among six European PRCVs and five coronaviruses of the TGEV antigenic cluster, based upon their RNA sequences. The S proteins of the six PRCVs were found to have an identical deletion of 224 amino acids starting at position 21. The deleted area includes the antigenic sites C and B of the TGEV S glycoprotein.
The significance of this virus as a pathogen is still unclear. Although it was first considered as non-pathogenic, subsequent investigations by A. Jestin et al., Recueil de Medecine Veterinaire, 163, 567 (1987); C. Duret et al., ibid., 164, 221 (1988); D. O'Toule et al., Res. Vet. Sci., 47, 23 (1989) and A. P. Van Nieuwstadt et al., Veterinary Record, 124, 43 (1989) have linked PRCV with field outbreaks of respiratory disease and with pneumonic lesions following an experimental infection.
Porcine respiratory coronavirus was first isolated in the United States in 1989. This isolate was designated as ISU-1-PRCV or Ind-89. Subsequently, ISU-2-PRCV was isolated from a herd in North Carolina. The herds involved seroconverted to TGEV without showing clinical signs of respiratory or enteric disease. See, H. T. Hill et al., Proc. Am. Assoc. Swine Prac., p. 333-335, (1989). ISU-1-PRVC replicated in the lungs of experimentally inoculated neonatal pigs but did not cause significant respiratory lesions. See, R. D. Wesley et al., J. Vet. Diag. Invest., 2, 312 (1990).
Therefore, a need exists for the isolation and characterization of novel pathogenic strains of TGEV, particularly with respect to pathogenic strains which may have evolved in the United States. Since it has been proposed by L. Enjuanes et al, in Coronaviruses, S. G. Siddell, ed., Plenum, N.Y. (1992) that PRCV behaves as a natural vaccine against TGEV, such PRCV strains could provide the basis for vaccines against both TGEV and pathogenic PRCV.